What happened

Shares of Mastercard (MA 0.45%) stock closed 5% higher on Monday -- Cyber Monday, to be precise, and therein lies half of the reason for the rise.

According to media and analyst reports, this year's Black Friday and Cyber Monday shopping spectaculars set new records for consumer spending, and for online purchases in particular. Friday alone, consumers spent more than $6.2 billion on online purchases, up 24% year over year. And the day before -- not even Black Friday -- consumers shelled out $3.7 billion in online shopping on Thanksgiving Day, up 28% year over year. Monday's totals aren't out yet, but with $3 billion in spending expected, Cyber Monday shopping could be up 25% year over year.

Hand holding blue credit card

Consumers put their credit cards to work on Black Friday. Image source: Getty Images.

So what

Obviously, it's hard to pay for online purchases with cash or check. That means for the most part, consumers were probably paying for their purchases with credit and debit cards, many bearing the Mastercard logo.

Result: Mastercard stock is going up because investors think it's likely that earnings will rise in tandem with rising sales.

Now what

Those earnings had better rise -- and by a lot -- if Mastercard investors are to be rewarded for their optimism. At current prices, Mastercard shares fetch nearly 39 times earnings, a high price even if analyst targets are met and Mastercard earnings grow at an expected 23% annualized rate over the next five years.

It would certainly help Mastercard to meet those targets if shoppers would give Mastercard a boost. And with sales growing at 20%-plus rates on each of the key shopping days so far, it looks like the 2018 holiday season is off to a running start.